It is known to use ultrasonic devices for creating a sealing joint between two or more flexible webs of sheet material. Such procedures can be used during the manufacture of packets for packaging materials therein. See, for example, U.S. Pat. No. 6,824,632. This document discloses a sonotrode and an opposed anvil. The webs to be joined together are passed between them in an aligned manner to compress the webs together, and at desired positions, the webs are then sealed together by activating the sonotrode to impart ultrasonic energy. See also WO-A-2004/000659, which discloses an apparatus that operates in a similar manner, having a rotating anvil (a roller) to help move the webs through the apparatus.
One development of the sonotrode/rotating anvil arrangement, such as that disclosed in WO-A-2004/000659, is illustrated in FIGS. 1 and 2. That development is to provide raised portions around the circumference of the roller anvil. They provide distinct areas of the roller against which the sonotrode can compress webs of material for welding those webs of material together. In this manner, switching on and switching off of the sonotrode between welds becomes less important since webs passing between the roller and the sonotrode will only be welded when compressed between a raised portion of the roller and the sonotrode. Therefore the sonotrode can be continuously driven by its ultrasonic actuator. This allows faster throughput. However, difficulties arise even with this arrangement. First of all, the maximum rotation speed of the rollers is governed by the available power of the sonotrode, and the demands on that sonotrode are rather high since the entire weld across and through the webs must be formed at substantially the same time. However, there is a limit as to how much ultrasonic energy can be delivered by a sonotrode actuator. Therefore, to prevent incomplete welds being formed across and through the webs as the webs pass between the sonotrode and a raised portion of the roller, it is necessary to limit how fast the roller is rotated. If the roller rotates too quickly, then each weld would be incomplete, i.e. a reliable seal would not be achieved because insufficient energy was delivered to the web during the short time that the weld line is located between the sonotrode and the raised portion of the roller. Secondly, due to the high demands that the raised portions place on the sonotrode, the loading profile for the sonotrode has significant spikes—see FIG. 4, whereby its working life can be inconveniently short.
It would be desirable, therefore, further to increase the throughput of ultrasonic web welding devices, whereby reduced manufacture times and/or increased production rates can be achieved.